taskqueue_start_threads() swallows lwkt_create failures and always returns 0 (mirror of DF-2850 in subr_taskqueue.c)

Summary

On lwkt_create error the loop kprintf's, NULLs the slot and continues, then unconditionally return 0. Total failure yields an unrunnable queue whose enqueues still succeed — every future taskqueue_drain on it sleeps forever (ta_pending never clears, no waker); partial failure silently degrades. Same defect class as DF-2850 (gtaskqueue) — this is the subr_taskqueue.c sibling. Fix: track the error, and on tq_tcount==0 free tq_threads and return it so callers abort.

Timeline

• 2026-09-02 Discovered during pass-2 audit of subr_taskqueue.c (GLM 5.3).